1. Field of the Invention
The present invention relates to an image reading apparatus, and more particularly, to an image reading apparatus for focusing image information of an original at a predetermined magnification on a surface of a recording medium (photosensitive drum) or on a surface of an image reading element (line sensor) by means of an imaging device to form or read the image information of the original.
2. Related Background Art
A variety of proposals have been made heretofore as to the image reading apparatus arranged to line-scan image information of an original or the like in the sub-scanning direction, to focus the image information on a surface of a line sensor (CCD) as an image reading element (solid state image sensing device), and to utilize output signals obtained from the line sensor to read the image information of the original or the like.
FIG. 1 is a schematic drawing of the major part of a conventional image reading apparatus of this kind. In FIG. 1, an original 58 on an original glass plate 51 is illuminated by an illuminating light source (linear light source) 55, a plurality of reflectors 56 provided around the illuminating light source 55, etc., the image information of the original 58 thus illuminated is guided through a slit 52 and then via first, second, and third mirrors 53a, 53b, 53c for scanning to be focused on the surface of line sensor 57 by an imaging lens 54, and the image information of original 58 is then read.
In the same drawing, a method for scanning (or reading) the image information of original 58 in the sub-scanning direction is, for example, one for moving the first, second, and third mirrors 53a, 53b, 53c for scanning in the direction of arrow A in the drawing (in the sub-scanning direction) while maintaining a constant relation among them. Scanning velocities of the respective scanning mirrors 53a, 53b, 53c at this time are set so that, for example supposing the velocity of the first mirror 53a is v, the scanning velocities of the second and third mirrors 53b, 53c are v/2.
In this construction, the image information formed on the surface of the line sensor 57 is converted (or photoelectrically converted) into an electric signal, and the electric signal is used for a variety of purposes as information of the image reading apparatus; for example, in some cases the electric signal is sent to an output device (not shown) to perform output of image information as a print output or in other cases the electric signal is sent to a memory device (not shown) to store the input image information.
The conventional image reading apparatus as described above, however, had the following problems.
(1) The first mirror 53a moving at the scanning velocity v is normally made in a unit form integrally with the illumination light source 55, the reflectors 56, etc., which is usually scanned (or moved) in the direction of arrow A in the drawing (in the sub-scanning direction). Accordingly, the conventional image reading apparatus must incorporate the illumination light source 55, the plural reflectors 56, and so on in the optical path between the surface of original 58 and the first mirror 53a, and therefore, the distance L becomes long inevitably between the surface of original 58 and the first mirror 53a, which results in increasing the size of the apparatus itself in the direction of the height.
(2) Since the illumination light source 55 is in the integral unit with the first mirror 53a and moves at high speed in the sub-scanning direction, a load is exerted on the tube itself, the filament, and so on of the illumination light source 55 by impact at start of scanning, vibration at stop of scanning, etc., which decreases the life of the light source.
(3) The first mirror 53a and the second and third mirrors 53b, 53c are constructed in respective units different from each other and are moved at the respective scanning velocities v, v/2. If there occurs a small angular error of mirror arrangement in each unit, there will occur a phenomenon that the illumination position on the surface of original 58 illuminated by the illumination light source 55 and reflectors 56 is different from the reading position on the surface of original 58 read on the surface of line sensor 57 through the first, second, and third mirrors 53a, 53b, 53c. If the distribution of luminous intensity in the sub-scanning direction on the surface of the original 58 illuminated by the illumination light source 55 and reflectors 56 at this time is a sharp distribution with little flat portions, an eclipse will occur by failing to obtain an effective quantity of light due to the deviation between the illumination position and the reading position.
(4) Since the width of illumination by the illumination light source 55 in the sub-scanning direction is finite, such a reflection phenomenon will occur that reflected light from portions other than the reading position returns to the illumination light source 55, reflectors 56, etc. adjacent thereto and is again reflected thereby so as to illuminate the original surface again. Since this reflection phenomenon is dependent upon the density of original, it will cause degradation of linearity of the level of quantity of reading light.